This invention relates generally to the field of telecommunications, and more particularly to a system and method for active filtering in a telecommunications network.
Many computing applications require the digital transmission of data to and from customer premises using the analog telephone system. Conventional modem technology is one such method. This technology transmits digital data over the telephone lines that connect customer premises with the local telephone company""s central office or an intermediate device such as a digital loop carrier (DLC). These telephone lines are typically twisted-pair copper wire and may be referred to as the xe2x80x9clocal loop.xe2x80x9d Conventional modem technology transmits the data in analog form over the local loop using the frequency band allocated for voice transmissions. However, the frequency range of the voice band, typically below 4 kHz, limits the speed at which a given volume of the data can be transmitted.
Digital subscriber line (DSL) technology, on the other hand, can apportion the transmission of signals over telephone lines into a voice channel and a digital data channel. For example, the voice channel may include signals below 4 kHz while the digital data channel may include signals above 25 kHz. Using this digital data channel, DSL provides a method of increasing the bandwidth of the existing telephone network infrastructure. Furthermore, DSL technology allows conventional plain old telephone service (POTS) devices that use the voice channel and DSL devices using the digital data channel to communicate simultaneously over the local loop. To accomplish this task, splitters or filters may be used to separate the two channels. Typically, a splitter or filter is placed at the central office, the customer premises, or both.
One filtering technique that may be used at the customer premises is a distributed filter architecture. In a distributed filter architecture, a low-pass filter is coupled between a customer premises wiring interface (such as an RJ-11 jack) and each telephone. For example, each telephone, facsimile machine, and other conventional telephony device may be coupled to a low-pass filter which is coupled to the customer premises telephone wiring. Each low-pass filter allows signals in the voice channel to travel to and from the conventional devices, while filtering out traffic and noise associated with devices using the digital data channel.
According to the present invention, disadvantages and problems associated with previous techniques for filtering in a telecommunications network have been substantially reduced or eliminated.
According to one embodiment of the present invention, a filtering device for active filtering at a customer premises includes an input interface that receives telephone signals and digital data signals communicated from a central office using a telephone line. The filtering device further includes an active low-pass filter that receives the telephone signals, the digital data signals, and electrical power from a power source. The active low-pass filter attenuates the digital data signals and passes the telephone signals using the electrical power. The filtering device also includes an output interface that receives the telephone signals from the active low-pass filter and communicates the telephone signals to a telephone.
The system and method of the present invention provide a number of important technical advantages. The present invention provides a filtering device for use in conjunction with DSL service at a customer premises that attenuates the high frequency digital data signals associated with the DSL service and prevents them from interfering with the telephones located at the customer premises. The filtering device may help prevent disruption of telephone service due to faults in the DSL equipment, such as a DSL modem. Furthermore, the filtering device may prevent noise associated with the digital data signals from unduly interfering with the analog telephone signals used by POTS telephones or the digital telephone signals used by ISDN telephones, while preventing higher frequency interference or noise associated with the telephones from unduly interfering with the digital data signals.
These functions are performed while reducing or eliminating losses in telephone signals that have been characteristic of previous passive filtering devices, particularly losses created when installing more than three to five of these passive filtering devices at a customer premises. Unlike passive filtering devices, a virtually unlimited number of filtering devices constructed according to the present invention may be implemented at a customer premises without creating unacceptable losses. Furthermore, unlike previous passive filtering devices, numerous filtering devices constructed according to the present invention may be implemented without creating interference between the telephone signals (POTS, ISDN, or otherwise) and the digital data signals (which may cause loss of data in the digital data signals). In addition, the present invention provides filtering devices that provide high order filtering without unduly degrading performance. To obtain similar order filtering using passive filtering devices, a relatively large number of cascaded filters are required, resulting in undesirable signal loss and interference. Other technical advantages are readily apparent to those skilled in the art.